The parameters of monoclinic unit cell of TlInSeS layered crystals were determined from X-ray powder diffraction study. The optical properties of TlInSeS have been investigated by means of transmission and reflection measurements in the wavelength range of 500-1100 nm. The optical indirect transitions with band gap energy of 2.05 eV and direct transitions with band gap energy of 2.21 eV were found by means of the analysis of the absorption data at room temperature. Transmission measurements carried out in the temperature range of 10-300 K revealed that the rate of change of the indirect band gap with temperature is gamma = -4.7 x 10(-4) eV/K. The dispersion of the refractive index is discussed in terms of the single oscillator model. The refractive index dispersion parameters: oscillator wavelength and strength were found to be 2.78 x 10(-7) m and 1.21 X 10(14) m(-2), respectively.